Letter pubs.acs.org/OrgLett
Rhodium-Catalyzed Asymmetric [2 + 2 + 2] Cycloaddition of 1,6Enynes with Cyclopropylideneacetamides Soichi Yoshizaki,† Yu Nakamura,‡ Koji Masutomi,† Tomoka Yoshida,‡ Keiichi Noguchi,§ Yu Shibata,† and Ken Tanaka*,†,‡ †
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, and §Instrumentation Analysis Center, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan S Supporting Information *
ABSTRACT: It has been established that a cationic rhodium(I)/H8−BINAP complex catalyzes the asymmetric [2 + 2 + 2] cycloaddition of 1,6-enynes with cyclopropylideneacetamides to produce spirocyclohexenes in excellent enantioselectivity with retaining cyclopropane rings.
T
Scheme 1
he transition-metal-catalyzed asymmetric [2 + 2 + 2] cycloaddition1 of 1,6-enynes with unsaturated compounds is a useful and straightforward method for construction of chiral bicyclic scaffolds.2,3 The Evans 2a and Shibata2b groups independently achieved this transformation by using alkynes as coupling partners and cationic rhodium(I)/axially chiral biaryl bisphosphine complexes as catalysts.4 These pioneering works enabled the synthesis of chiral bicyclic cyclohexadienes with one stereogenic center with high enantioselectivity. The next challenge is the use of alkenes as coupling partners to produce chiral bicyclic cyclohexenes with two stereogenic centers with high diastero- and enantioselectivity.5 Recently, our research group achieved this transformation by using acrylamides as alkenes and a cationic rhodium(I)/H8−BINAP complex as a catalyst.3,6 Subsequently, our research group reported that the cationic rhodium(I)/H8−BINAP complex catalyzes the [3 + 2 + 2] cycloaddition of 1,6-diynes with cyclopropylideneacetamides7 to produce cycloheptadienes through cleavage of cyclopropane rings, on the contrary, a cationic rhodium(I)/BINAP complex catalyzes the asymmetric [2 + 2 + 2] cycloaddition of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides to produce spirocyclohexadienes8 with retaining cyclopropane rings (Scheme 1).9,10 In this letter, we disclose the rhodium-catalyzed asymmetric [2 + 2 + 2] cycloaddition of 1,6-enynes with cyclopropylideneacetamides leading to spirocyclohexenes with retaining cyclopropane rings. Interestingly, when using an aliphatic alkene and diethyl acetylenedicarboxylate in place of the 1,6-enyne, the rhodium-catalyzed linear [3 + 2 + 2] trimerization proceeded to give a triene through cleavage of the cyclopropane ring (Scheme 2). © XXXX American Chemical Society
Scheme 2
We first examined the reaction of 1,6-enyne 1a and N-methylN-phenylcyclopropylideneacetamide (2a, 1.1 equiv) at room temperature in the presence of 20 mol % of the cationic rhodium(I)/(S)-H8−BINAP complex (Table 1, entry 1). Pleasingly, [2 + 2 + 2] cycloaddition product 3aa was obtained with excellent ee value (Table 1, entry 1), while a significant amount of a homo-[2 + 2 + 2] cycloaddition product from 1a was Received: November 26, 2015
A
DOI: 10.1021/acs.orglett.5b03387 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 3a
Table 1. Optimization of Reaction Conditions for RhodiumCatalyzed Asymmetric Cycloaddition of 1a with 2aa
entry
ligand
catalyst (mol %)
1a/2a (equiv)
time (h)
3aa (%) yieldb (% ee)
1 2 3 4 5 6 7 8
(S)-H8−BINAP (S)-H8−BINAP (S)-H8−BINAP (S)-BINAP (S)-Segphos (S,S)-DIOP (S)-H8−BINAP (S)-H8−BINAP
20 20 20 20 20 20 15 10
1.0/1.1 1.0/2.0 2.0/1.0 2.0/1.0 2.0/1.0 2.0/1.0 2.0/1.0 2.0/1.0
24 24 24 24 24 24 72 72
54 (99) 45 (99) 69 (99) 59 (97) 42 (99) 0 46 (99) 37 (99)
a
[Rh(cod)2]BF4 (0.010−0.020 mmol), ligand (0.010−0.020 mmol), 1a (0.10−0.20 mmol), 2a (0.10−0.20 mmol), and CH2Cl2 (2.0 mL) were used. bIsolated yield.
generated as a byproduct. In order to suppress the undesired homo-[2 + 2 + 2] cycloaddition, the amount of 2a was increased to 2 equiv, but the reaction rate decreased significantly (entry 2).11 Pleasingly, increasing the amount of 1a increased the yield of 3aa with maintaining the reaction rate (entry 3). Screening of bisphosphine ligands (Figure 1) revealed that the use of biaryl
[Rh(cod)2]BF4 (0.0075−0.010 mmol), (S)-H8−BINAP (0.0075− 0.010 mmol), 1a−h (0.20 mmol), 2a−f (0.10 mmol), and CH2Cl2 (2.0 mL) were used. The cited yields were of isolated products. a
Figure 1. Structures of chiral bisphosphine ligands.
hexene 3ac. Among cyclopropylideneacetamides examined, N,Ndimethylcyclopropylideneacetamide 2c showed the highest reactivity, and the catalyst loading could be reduced to 15 mol % without loss of the product yield. With respect to 1,6-enynes, not only methyl (1a) but also ethyl (1b) substitution at the alkene moiety was tolerable. However, 1,6-enyne 1c, possessing the monosubstituted alkene moiety, and terminal 1,6-enyne 1d reacted with 2b and 2c, respectively, to give spirocyclohexenes 3cb and 3dc in low yields.12 Not only tosylamide- (1a−e) but also nosylamide- (1f) and benzamide- (1g) linked 1,6-enynes could be employed for this reaction, while malonate- and oxygenlinked 1,6-enynes 1g and 1h, which are suitable substrates for acrylamides, were failed coupled with 2a.12 Finally, construction of two quaternary carbon centers was attempted, while the desired spirocyclohexene 3af was not obtained at all. Importantly, excellent ee values (92 to >99%) were observed in all products. The relative and absolute configurations of (+)-3ad were unambiguously determined by an X-ray crystallographic analysis. A possible mechanism for the formation of 3 and 5 is shown in Scheme 4. Enyne 1 reacts with rhodium to generate rhodacyclopentene A. Regioselective insertion of alkene 2 into A generates rhodacycle B. Reductive elimination affords
bisphosphine ligands, possessing smaller dihedral angles than H8−BINAP, decreased the yields of 3aa (entries 4 and 5), and the use of a nonbiaryl bisphosphine ligand failed to furnish 3aa (entry 6). Unfortunately, decreasing the catalyst loadings to 15 or 10 mol % decreased the yield of 3aa (entries 7 and 8). Importantly, in all entries, [3 + 2 + 2] cycloaddition product 4aa was not detected at all. This feature is in sharp contrast to the reactions of 1,6-diynes with cyclopropylideneacetamides, which furnishes [3 + 2 + 2] cycloaddition products as major products (Scheme 1).9 The generality of the reaction with regard to both cycloaddition partners was tested as shown in Scheme 3. Not only Nmethyl-N-phenyl- (2a) but also N,N-diphenyl- (2b) and N,Ndialkyl- (2c,d) cyclopropylideneacetamides reacted with 1a to give spirocyclohexenes 3aa−3ad in good yields with excellent ee values. However, cyclopropylideneacetate 2e failed to react with 1a. Substituents on cyclopropylideneacetamides 2 affected the product yield and distribution in this process. Increasing the coordination ability of 2 (2b < 2a < 2c) increased the yields of spirocyclohexenes 3 (3ab < 3aa < 3ac). Interestingly, the reaction of 1a with highly coordinative N,N-dimethylcyclopropylideneacetamide 2c afforded triene 5ac as well as spirocycloB
DOI: 10.1021/acs.orglett.5b03387 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 4
Scheme 6
cycloaddition of 1,6-enynes with cyclopropylideneacetamides to produce spirocyclohexenes in excellent enantioselectivity with retaining cyclopropane rings; on the contrary, the reaction of 1tetradecene, diethyl acetylenedicarboxylate, and the cyclopropylideneacetamide affords a linear [3 + 2 + 2] trimerization product through cleavage of the cyclopropane ring.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b03387. X-ray crystallographic file (CIF) Experimental procedures and compound characterization data (PDF)
spirocyclohexene 3. The amide carbonyl oxygen coordinates to rhodium, and the cyclopropane moiety located at α position with respect to rhodium would suppress β-hydrogen and carbon eliminations, respectively. When using highly coordinative N,Ndimethylcyclopropylideneacetamide 2c, the alkyne moiety of 1 reacts with 2 and rhodium to generate rhodacyclopentene C.13 Insertion of the alkene moiety of 1 into C generates rhodacycle D. β-Carbon elimination affords rhodacycle E and subsequent βhydrogen elimination affords rhodium hydride F. Reductive elimination affords triene 5. As shown in Scheme 1, the reaction of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides afforded [2 + 2 + 2] cycloaddition products with retaining cyclopropane rings.9 Thus, the reaction of 1-tetradecene (6), diethyl acetylenedicarboxylate (7), and cyclopropylidene-acetamide 2c was examined (Scheme 5).13 Interestingly, not the spirocyclohexene but triene 8 was obtained in low yield with good ee value through cleavage of the cyclopropane ring.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported partly by ACT-C from the Japan Science and Technology Agency (Japan), and a Grant-in-Aid for Scientific Research (No. 25105714) from the Ministry of Education, Culture, Sports, Science, and Technology (Japan). We thank Mr. Hiroshi Kanno (Tokyo University of Agriculture and Technology) for valuable assistance. K.M. thanks JSPS research fellowship for young scientists (No. 27·7947). We are grateful to Takasago International Corporation for the gift of H8−BINAP and Segphos, and Umicore for generous support in supplying the rhodium complex.
Scheme 5
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REFERENCES
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A possible mechanism for the formation of 8 is shown in Scheme 6. Similar to the proposed mechanism of our previously reported cationic rhodium(I)/(R)-BINAP complex-catalyzed asymmetric linear cross-trimerization of alkenes, acetylenedicarboxylates, and acrylamides,6i 6 and 7 react with rhodium to generate rhodacyclopentene G. Regioselective insertion of 2c into G generates rhodacycle H. β-Carbon elimination affords rhodacycle I and subsequent β-hydrogen elimination affords rhodium hydride J. Reductive elimination affords triene 8. In summary, we have established that a cationic rhodium(I)/ H8−BINAP complex catalyzes the asymmetric [2 + 2 + 2] C
DOI: 10.1021/acs.orglett.5b03387 Org. Lett. XXXX, XXX, XXX−XXX
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D
DOI: 10.1021/acs.orglett.5b03387 Org. Lett. XXXX, XXX, XXX−XXX
